1. Field of the Invention
The invention relates to thermal waterless lithographic printing plates comprising layers of inherent near infrared absorbing polymers for computer-to-plate and digital-offset-press technologies. More specially, this invention relates to thermal waterless lithographic printing plates, which can be imaged with near infrared laser light and which do not require post chemical processing step.
2. The Prior Art
Thermal waterless lithographic printing plates are known. For example, U.S. Pat. Nos. 5,310,869 and 5,339,737 describe thermal waterless lithographic printing plates comprising an ink-repelling layer overlying a near infrared absorbing imaging layer. The ink-repelling layer is transparent to radiation and comprises mainly cross-linked silicone polymers. The near infrared absorbing imaging layer contains binder resins and near infrared absorbing materials, such as carbon black and molecular dyes. These thermal waterless lithographic printing plates require high doses of laser energy to ablate the near infrared absorbing layer and weaken the adhesion of the ink repelling cross-linked silicone polymer layer. In addition, the exposed area of the plate must be removed during a further chemical processing step to become an image area.
U.S. Pat. No. 5,379,698 also describes thermal waterless lithographic printing plates, which comprise ink repelling cross-linked silicone polymers overlying a thin metallic or metal oxide film of titanium deposited on a substrate as a laser imaging layer. In a similar technology, U.S. Pat. No. 5,487,338 teaches to use an infrared reflective layer situated below the near infrared absorbing layer. Manufacturing of such printing plates requires vacuum deposition of the corresponding metals. Hence it is very expensive.
WO9831550, WO9700175 and WO9401280 also describe thermal waterless lithographic printing plates, which comprise a layer of ink repelling cross-linked silicone polymers overlying a near infrared absorbing imaging layer containing binder resins and near infrared absorbing pigments, dyes or thin metal films. Again, such thermal waterless lithographic printing require high laser energy doses for imaging.
WO9706956 also describes thermal waterless lithographic printing plates, which comprise a near infrared absorbing layer containing binder resins and near infrared absorption dyes or pigments, and a overlying transparent hydrophobic layer containing fluorinated polymeric materials soluble in fluorinated solvents. Upon exposure to near infrared laser radiation, the exposed area is ablated and accepts ink, while the non-exposed area still repels ink. One drawback of such plates is that the non-exposed area is sensitive to handling and easily becomes dirty on press.
EP0764522 also provides a thermal waterless printing plate containing a near infrared transparent cross-linked silicone polymer ink repelling layer and a near infrared absorbing imaging layer. The ink repelling layer and near infrared absorbing imaging layers contain cross-linked functionality, which form interlayer cross-linked bonds to increase the run length on press. Such printing plate requires high laser energy doses for imaging and requires a chemical processing step.
WO9911467 also provides a thermal waterless lithographic printing plate, comprising a layer of ink repelling cross-linked silicone polymer overlying a near infrared absorbing imaging layer containing polyurethane resins and near infrared absorption dyes. Although, such printing plate exhibits faster laser imaging speed, they are very sensitive to the different developers used in the final chemical processing step.
Thus there remains a need for an improved thermal waterless lithographic printing plate which overcomes the drawbacks of the prior art.
The main objects is to provide lithographic printing plate coating compositions which combine the advantages of: long-life printing plates, absence of phase separation of the overlaid coatings, easily manufactured and inexpensive coating formulations, coatings which may be precisely and rapidly imaged with laser accuracy.